The present invention relates to an iron or steel component, subject to friction, for a rotary piston machine, particularly to its double-walled side member, central portion or trochoid housing made of cast iron. The present invention furthermore relates to a process of producing such components subject to friction.
In the operation of such rotary piston machines the maximum achievable operational life is essentially determined by the components subject to friction. These components possess large flat surface areas over which the rotating piston wipes with its sealing strips so that a comparatively large frictional stress occurs. Since the rotary piston edges or sealing strips wiping over these surface areas must ensure a gas-tight seal, only small wear of these areas is admissible. Accordingly, it has for some time been attempted to make these components, or at least their surface areas subject to friction, as wear-resistant as possible. However, this problem is difficult to solve because it involves complex components, particularly because the two-walled side or central portions are provided with a large number of bores and connections. If at all, such double-walled cast-iron components with major wiped surfaces subject to friction may, however, be treated with the previously usual processes for surface hardening only with great difficulty.
Hardening of the wiped areas of such components has so far been effected with varying results in well-known metal spraying processes which, however, constitute a most expensive treatment which is moreover dangerous owing to possible adverse effects on health. Apart from the fact that the raw materials employed, such as molybdenum, can often only with difficulty be obtained in the quantities required for the treatment of a large number of components, the wiped surfaces provided with such a coat must subsequently be reground, which results in a further cost increase. Reworking such coated areas following a certain period of operation of the rotary piston machines has so far not been possible.
Again, the so-called induction hardening of such wiped areas is possible, but in view of the great stresses arising in the operation of such rotary piston machines such hardened wiped surfaces are not sufficiently wear-resistant in order to ensure satisfactory operational life. The deformations occurring in such a treatment are in addition so pronounced that they require regrinding and can hardly be eliminated nonetheless.
Experiments made with salt-bath nitriding and also with thermal gas nitriding produced no acceptable result because the hardening process was there not limited to the wiped surfaces proper but resulted in subjecting the components in question to nitriding as a whole. In view of the complex double-walled castings with their irregular bores, experience has shown that deformation occurs so that the desired high precision of the wiped areas is no longer ensured. In addition, the absorption of certain quantities of nitrogen in the base material results in a change in size, which cases substantial difficulties in respect of preserving the necessary dimensional accuracy. Moreover, residues of the nitriding bath, which can virtually not be removed, are formed particularly in salt-bath nitriding complex hollow components.
Despite the failure of the common nitriding processes, applicant has undertaken to achieve the desired properties using the ionitriding process. In so doing, it has surprisingly found that it is possible in so-called ionitriding by means of an electrical glow discharge in a gas atmosphere containing nitrogen, to produce, with adequate control of the ionitriding process, such components which have outstandingly good running properties without necessitating reworking of the components that have previously been processed to their final dimensions.
The present invention relates to such an iron or steel component, which is subject to friction, for a rotary piston machine, particularly for its double-walled side unit, central unit or trochoid housing made of cast iron, which is characterized in that the surfaces limited to the wiped areas are hardened by ionitriding with an iron nitride zone reduced to less than 10 microns thickness and with a surface roughness of a maximum of R.sub.2 = 0.8 micron, with a corrosion resistance relative to combustion residues of fuels increased by at least 300 percent, with a thermal resistance up to 500.degree.C over long periods or up to 800.degree.C over short periods, and with a ductility reduced by a maximum of 10 percent as compared with the basic material.
The invention furthermore relates to a method of producing the component subject to friction according to this invention, ionitriding being performed by means of an electrical glow discharge in a gas atmosphere containing nitrogen at an elevated temperature in a treatment chamber, characterized in that the said component is provided with a coating inhibiting the electrical glow discharge on the surfaces outside the wiped surfaces proper, then ionitriding the surfaces of the wiped areas after inserting the component in the treatment chamber for at least five hours at a gas pressure of 0.1 - 5 mmHg and an operating voltage above 500 Volts, the said operating voltage and the gas pressure being so controlled that the iron nitride zone is partly removed by spattering metal particles from the surface and the thickness of the zone thus held below 10 microns.